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Using Sound Waves to Produce Optical Isolators

These devices may solve data capacity challenges for circuits used in computing and communications.

University of Illinois Urbana-Champaign, Champaign, Illinois

There are challenges involved when using magnetically responsive materials to achieve the one-way flow of light in a photonic chip, including the ability to place compact magnets on a chip. In addition, the necessary materials are not yet available in photonics foundries, creating the need for a better approach that uses only conventional materials and avoids magnetic fields.

Sound waves can be used to produce ultraminiature optical diodes that are tiny enough to fit onto a computer chip. These optical isolators may help solve major data capacity and system size challenges for photonic integrated circuits — the light-based equivalent of electronic circuits used for computing and communications.

Isolators are nonreciprocal or “oneway” devices similar to electronic diodes. They protect laser sources from back reflections and are necessary for routing light signals around optical networks. Today, the dominant technology for producing such nonreciprocal devices requires materials that change their optical properties in response to magnetic fields.

The minuscule coupling between light and sound was used to provide a unique solution that enables nonreciprocal devices with nearly any photonic material. Still, the physical size of the device and the availability of materials are not the only problems with the current state of the art. Laboratory attempts at producing compact magnetic optical isolators have always been plagued by large optical loss. A solution is needed that provides enough bandwidth to be comparable to the traditional magnetic technique.

The new device is only 100 × 200 microns in size — about 10,000 times smaller than a square centimeter — and made of aluminum nitride, a transparent material that transmits light and is compatible with photonics foundries. Sound waves are produced in a way similar to a piezoelectric speaker, using tiny electrodes written directly onto the aluminum nitride with an electron beam. These sound waves compel light within the device to travel only in one direction. The magnetless isolator surpassed gigahertz bandwidth.

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